Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
Unit 4
Water for a Desert City
In this unit, you will
• Identify water-related challenges faced by desert and western cities.
• Examine the importance of maintaining a water balance.
• Determine the economic and environmental consequences of
excessive groundwater pumping.
• Investigate water-use patterns and the importance of water
conservation.
• Develop a plan to help Tucson, Arizona meet its future water needs.
L. P. Kendall, The SAGUARO Project
Downtown Tucson, Arizona and the Catalina Mountains.
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Unit 4 – Water for a Desert City
Data Detectives: Where’s the Water?
Warm-up 4.1
The four great deserts
of the Southwest
Great Basin
Mojave
Sonoran
Chihuahuan
Figure 1. Red dots represent cities
in the four major deserts of the
southwestern U.S. and Mexico.
Unit 4 – Water for a Desert City
Living in a desert
Desert regions are characterized by low annual precipitation and a high
evaporation rate. These regions, covering approximately  percent
of the land on Earth, may be large expanses of shifting sand with few
plants, or rugged mountainous regions colonized by many plant species.
In the United States, the four large deserts of the Southwest — the
Chihuahuan, Great Basin, Mojave, and Sonoran deserts — contain
many cities with significant populations (Figure ). One of these cities
is Tucson, the second largest metropolitan area in Arizona, with a
population of over ,. This region was originally occupied by
Native Americans and later by Spanish settlers who established missions
and military posts beginning in . After Arizona became part of the
United States in , the population of Tucson grew as people realized
the potential for mining and agriculture. The continued population
growth of the Tucson area has had important consequences for the
environment. The demand for more water is met by drilling new wells
and importing surface water from distant sources.
. Which kind of water reservoir is tapped by pumping water from
wells?
Hint for question 1
Refer back to Warm-up 1.1 and
Investigation 1. 2 in Unit 1 to
refresh your memory of all the
water reservoirs on Earth.
. Speculate on the environmental consequences of removing water
from this reservoir faster than it is replenished by the hydrologic
cycle.
. Do you think that issues of water supply and demand are unique
to desert cities and towns? Explain how the problems of obtaining
adequate water for desert inhabitants might apply to your city or
town.
Living in a desert
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Unit 4 – Water for a Desert City
The Santa Cruz River was a critical water source for early Tucson
inhabitants. Archaeologists have found evidence of agricultural
settlements along the Santa Cruz dating back to approximately  B.C.
Compare the two photographs on page  taken of the west branch
of the Santa Cruz River. Both photographs were taken from the same
location,  years apart.
. Describe the major differences in the environment between the
two photographs.
. What do you think might have caused the changes?
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
Photo by Walter Hadsell, courtesy of Arizona Historical Society
Catalina Mountains
Rincon Mountains
University of Arizona
Main branch, Santa Cruz River
Wes
t bra
nch,
Sant
a Cru
z R iv
er
Figure 2. West branch of the Santa Cruz River in 1904. The city of Tucson, Arizona is in the background.
Dominic Oldershaw, USGS
Catalina Mountains
Rincon Mountains
University of Arizona
Main branch, Santa Cruz River
Figure 3. View from the same location in 2000. The west branch of the Santa Cruz River has been filled
in, but the main channel is still visible, above center.
Living in a desert
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Living in a desert
Unit 4 – Water for a Desert City
Data Detectives: Where’s the Water?
Investigation 4.2A
Unit 4 – Water for a Desert City
Water in the balance
Maintaining a balance between the water we use and the water supply
that is replenished by precipitation is the key to ensuring that cities and
agriculture continue to thrive. This is a particularly demanding task in
desert regions. The deserts of the American Southwest contain bustling
cities and an extensive network of farms and ranches that depend on
water for survival.
Aquifer — a water-bearing rock
formation.
Tucson, Arizona is a desert city that relies on groundwater withdrawn
from the underlying aquifer to meet almost all its water needs.
Therefore, to maintain a water balance, we must understand when,
where, and how the water in an aquifer is replenished. In this
investigation, you will examine the factors critical to replacing water in
an aquifer.
Precipitation patterns in the Tucson Basin
Precipitation events provide most of the water that replenishes
aquifers. Therefore, examining precipitation patterns is the first step to
understanding aquifer recharge.
Launch ArcMap, then locate and open the ddww_unit_.mxd file.
Refer to the tear-out Quick Reference Sheet located in the Introduction to
this module for GIS definitions and instructions on how to perform tasks.
In the Table of Contents, right-click the Precipitation Patterns
data frame and choose Activate.
Expand the Precipitation Patterns data frame.
This data frame shows a shaded relief image of the Tucson area as well as
the city streets.
Turn on the Precipitation Zones layer.
Drainage basin — an area in
which runoff flows downhill to a
common point, such as a lake; or
to a common channel, such as a
river or stream.
The colors that are displayed in this layer correspond to the amount of
annual precipitation received by different areas, averaged over a -year
period. The heavy dashed line marks the approximate boundary of the
Tucson Basin, including the slopes of the mountains that drain into the
basin.
. Use the legend to determine the highest precipitation range in the
Tucson Basin (the area inside the heavy dashed line).
Water in the balance
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
. What color represents the highest precipitation range found in the
Tucson Basin?
. Is the amount of annual precipitation the same across the Tucson
Basin? Explain your observations.
Turn on the Weather Stations layer.
This layer shows the location of weather stations within and outside the
Tucson Basin.
Click the Identify tool .
In the Identify Results window, select the Weather Stations layer
from the drop-down list of layers.
Using the Identify tool , determine which weather stations
inside the Tucson Basin (inside the dotted line on the map) receive
the highest and the lowest amounts of annual precipitation. You
may need to scroll down to see the annual precipitation inside the
Identify Results window.
. In Table , record the location of weather stations that receive the
highest and lowest amount of annual precipitation in the Tucson
Basin. Include the amount of annual precipitation they receive and
the month they receive the most precipitation.
Table 1 — Highest and lowest annual precipitation in the Tucson Basin
Location of
station
Annual
precipitation
cm/yr
Month of
highest
precipitation
Station with the
highest precipitation
Station with the
lowest precipitation
. Do the data you entered in Table  support your answer to
question ? Explain.
Close the Identify Results window.
Turn off the Precipitation Zones and Weather Stations layers.
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Unit 4 – Water for a Desert City
Aquifer recharge
Precipitation is continually replacing, or recharging groundwater that
people remove from the aquifer. Of course, not all precipitation ends
up in the aquifer. As you learned in previous activities, precipitation is
diverted through different parts of the hydrologic cycle when it reaches
land.
Examine the hydrologic cycle diagram (Figure ) below to explore what
can happen to precipitation when it reaches the ground.
Condensation
Precipitation
Transpiration
Evaporation
Runoff
River
Water table
Infiltration
Figure 1. Land processes of the hydrologic cycle.
. Which processes in the hydrologic cycle recharge the aquifer?
Natural recharge
Stream flow
Precipitation infiltrates basin soils and, to a lesser extent, joints,
fractures, and faults in the bedrock of the surrounding mountain
ranges. If precipitation falls where the ground is too wet or where
water is unable to penetrate the surface, that water becomes runoff and
joins streams and rivers. In the Tucson Basin, streams and rivers are
usually dry because the water table, or the top boundary of the aquifer,
lies below the river bottoms (Figure ). After precipitation events,
these water channels fill with runoff and merge together as they flow
downstream and eventually drain into the Santa Cruz River.
Turn off the Streets layer.
Turn on the Streams layer.
The V shape, which forms
where streams merge, points
downstream.
This layer illustrates the network of streams and rivers that flow through
the Tucson Basin. The V shape that is formed where two streams merge
always points downstream, as shown at left.
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Data Detectives: Where’s the Water?
Gauging station
One type of gauging station is
called a stilling well, illustrated
below.
Unit 4 – Water for a Desert City
. In which direction does water flow out of the Tucson Basin?
(North is “up.”)
Turn on the Gauging Stations layer.
recorder
float
A river or stream’s rate of flow, usually expressed in m/sec (or ft/sec),
is called its discharge. Discharge is measured by instruments at gauging
stations. This layer shows the locations of gauging stations in the
Tucson Basin. Data from these stations will help you determine whether
discharge increases or decreases as water moves downstream.
intakes
Click the Identify tool . (Note: You may need to click the
Pointer tool before clicking the Identify tool, in order to open
the Identify Results window again.)
In the Identify Results window, select the Gauging Stations layer
from the drop-down list of layers.
Use the Identify tool to find the name of each gauging station.
well
Stream water enters the well
through intake pipes. The water
level in the well is the same as
the water level in the stream. As
the water level rises or falls, the
float in the well also rises or falls. A
cable attached to the float drives
a device that records the water
level or transmits it to a satellite
for recording at a central location
(adapted from USGS).
. Label the gauging stations in the boxes of Diagram .
Diagram 1 — Rillito Creek gauging stations
Santa Cruz River
. In the circles, indicate the number of streams that enter Rillito
Creek between gauging stations.
. Do you expect the discharge to increase or decrease as the water
flows out of the Tucson Basin? Explain your answer.
Close the Identify Results window.
Click the Media Viewer button and choose the Discharge
Hydrograph movie.
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Data Detectives: Where’s the Water?
Spiky data
Unit 4 – Water for a Desert City
This movie shows the amount of discharge flowing through the four
gauging stations on the Rillito Creek and Santa Cruz River as surface
waters leave the Tucson Basin.
Watch the movie several times to determine whether discharge
increases or decreases between each of the stations.
The data in the Discharge
Hydrograph movie appear more
“spiky” than the precipitation data
you looked at earlier, because
they are daily measurements
rather than monthly averages.
Furthermore, the gauging station
data are for a single year (1998)
rather than a 30-year average.
Watch a flash flood!
. Look at the discharge between the months of February and May.
How does the discharge change between the following gauging
stations? (Does it increase, decrease, or remain constant?)
a. Tanque Verde Creek and Rillito Creek at Dodge Blvd.
b. Rillito Creek at Dodge Blvd. and La Cholla Blvd.
. Compare your observations in question  to your prediction in
question . Explain any differences between your prediction and
observations.
©1990 John Smith, USDA/ARS/Southwest Watershed Research Center
You may have noticed that there is a small, but measurable discharge at
the Santa Cruz River gauging station throughout the year, as indicated
by the blue dashed line. This line represents base flow, which is the result
of pumping treated wastewater from the sewer system into the river
channel.
. What is the approximate base flow, in m/sec, at the Santa Cruz
River station?
. What might be the benefit of adding this water to the river
system?
To view a movie of a flood
caused by summer thunderstorm
precipitation runoff, click the
Media Viewer button
and
choose Flash Flood movie.
Close the Media Viewer window.
Turn off the Streams and Gauging Stations layers.
Estimating annual recharge
Next you will estimate the mean amount of water recharged each year in
the Tucson Basin aquifer.
Turn on the Recharge Regions layer.
The Recharge Regions layer shows the three main recharge regions of
the Tucson Basin. Only the portion of the mountain slopes that drain
into Tucson Basin are included.
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
Click the Identify tool . (Note: You may need to click the
Pointer tool before clicking the Identify tool, in order to open
the Identify Results window again.)
In the Identify Results window, select the Recharge Regions layer
from the drop-down list of layers.
Using the Identify tool , click in each recharge region to gather
information to record in Table  below.
. Record the mean annual precipitation and the area for each
recharge region in Table .
Table 2 — Calculating recharge in the Tucson Basin
Recharge area name
Mean
precipitation
Area
Precipitation
volume
Recharge volume
m
m2
m3
m3
click in region using
the Identify tool
click in region using
the Identify tool
= area x mean precipitation = precipitation volume x 0.05
Tucson Mountains
Central Basin
Catalina – Rincon Mountains
Total (add values in each column)
. Calculate the volume of precipitation that falls in each region
by multiplying the mean precipitation by the area. Record your
results in the Precipitation volume column of Table .
Research has shown that only about  percent of the precipitation that
falls in the Tucson Basin recharges the aquifer.
. Calculate the volume of water recharged in each region of the
Tucson Basin by multiplying the precipitation volume by .
(%) and record your results in the Recharge volume column of
Table .
. Calculate and record the Total Area, Precipitation volume, and
Recharge volume by adding the values in each column of Table .
. Based on what you know about the hydrologic cycle and the
climate of the southwestern U.S., why do you think the recharge
rate (%) is so low in the Tucson Basin?
Close the Identify Results window.
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Data Detectives: Where’s the Water?
The Colorado River —
a desert lifeline
Originating in the Rocky
Mountains, the Colorado River
is a lifeline for the southwestern
United States. Most of the territory
the river crosses on its way to the
Gulf of California is desert. Along
the way, water is withdrawn to
meet the needs of 20 million
people and to irrigate millions of
acres of farmland.
Acre-foot — the amount of water
needed to cover one acre of land
to a depth of 1 foot. One acre-foot
equals about 1230 cubic meters
(or 325,851 gallons of water), the
amount used by an average family
of four in one year.
Unit 4 – Water for a Desert City
Artificial recharge
So far you have explored natural recharge of water to the aquifer by
precipitation. The aquifer can also be artificially recharged with treated
wastewater and by surface water imported from other areas. For
example, the Central Arizona Project (CAP), completed in  at a
total cost of $ billion, delivers approximately . billion cubic meters
(. million acre-feet) of Colorado River water to cities in central and
southern Arizona each year. The CAP canal carries open surface water
along a -km (-mi) trip through the desert from Lake Havasu on
the Colorado River to a point just north of Tucson, where it is recharged
into the aquifer. Pumping stations and pipelines provide some help
along the way to get across areas of higher elevation.
Collapse the Precipitation Patterns data frame.
Right-click the Supply and Demand data frame and choose
Activate.
Expand the Supply and Demand data frame.
This data frame shows Arizona counties, the CAP canal system, and
selected cities that receive CAP water.
Click the Identify tool .
In the Identify Results window, select the Arizona Cities layer
from the drop-down list of layers.
Using the Identify tool , click on Tucson (the southernmost
city) to gather information about the city.
Allotment — the volume of water
a city, state, or country (U.S. and
Mexico) is assigned each year
from the Colorado River.
. What is the annual allotment of CAP water delivered to Tucson in
cubic meters (m)?
. Using Table  on page , compare Tucson’s annual CAP
allotment with the annual recharge of the Tucson Basin.
. How do you think Tucson and the Tucson Basin Aquifer would
be affected if Tucson’s CAP allotment were reduced or cut off
completely?
Close the Identify Results window.
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
How is Tucson’s water supply used?
Like a bank account, maintaining a water balance requires that the water
removed from the aquifer be replaced. It is important, then, to keep
track of who is using water and how much is being used. In this section
you will look at how water is used in Pima County, of which Tucson is
the largest city.
Water is used for many different purposes beyond drinking and typical
household uses.
Public water supply — includes
water used in homes and
businesses, as well as public use in
parks and recreational facilities.
. List examples of how water is used in each of the following wateruse classifications.
a. Public water supply.
b. Mining.
c. Agriculture.
d. Industrial.
. Speculate on the percentage of groundwater designated for use in
the public water supply, mining, agriculture, and industry. Draw a
pie chart showing your estimates. Be sure to label each segment.
Turn off the CAP Canal layer.
Turn on the Water Usage layer.
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
This layer displays pie charts that show the breakdown of water usage for
each county in the state.
. Draw and label the actual pie chart for groundwater usage in Pima
County.
. How did your estimate compare to the actual chart? Which part of
your estimate was most in error?
Quit ArcMap and do not save changes.
Water in the balance
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Data Detectives: Where’s the Water?
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Water in the balance
Unit 4 – Water for a Desert City
Data Detectives: Where’s the Water?
Investigation 4.2B
Unit 4 – Water for a Desert City
Water in the balance
In addition to understanding when, where, and how the water in an
aquifer is recharged, we must also understand how water is used. In
this investigation, you will examine classifications of water use and how
factors such as population growth and climate conditions affect water
availability in Tucson, Arizona.
Tucson’s water balance sheet
As mentioned earlier, assessing Tucson’s water situation is similar
to maintaining a bank account. There are deposits: recharge from
precipitation, treated wastewater, and CAP water added into the aquifer.
There are also withdrawals from the aquifer: water used by the people
and businesses of Tucson. The goal, like balancing a checkbook, is
to make sure that there are enough water deposits into the aquifer to
replace the amount of water withdrawn from the aquifer.
In the Tucson Water Balance Sheet, you will balance the “water
checkbook” to see if there are enough deposits of surface water to match
the withdrawals of groundwater from the aquifer, based on the city’s
population in  and projected into the year .
Launch ArcMap, then locate and open the ddww_unit_.mxd file.
Refer to the tear-out Quick Reference Sheet located in the Introduction to
this module for GIS definitions and instructions on how to perform tasks.
In the Table of Contents, right-click the Supply and Demand data
frame and choose Activate.
Expand the Supply and Demand data frame.
Click the Media Viewer button and choose Tucson Water
Balance Sheet. Or, you can locate the Tucson Water Balance
Sheet Excel file in the Unit  folder and open it in Microsoft
Excel.
Per capita — per person. For
example, water use per capita
describes the amount of water
each person would use if Tucson’s
total water use was divided
equally among everyone living in
Tucson.
This spreadsheet allows you to adjust the rate of population growth, the
rate of increase in water usage (per capita per year), and the amount of
mean precipitation per year. By adjusting these variables, you can find
combinations of these factors that will result in a water balance.
Examine the Water Balance graph.
Water in the balance
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
. If growth and water usage (including the use of CAP water) in
Tucson continues at the current rate, will Tucson ever achieve a
water balance (as indicated by positive values on the graph labeled
“Water Balance”)?
. During which year will Tucson come closest to achieving a water
balance?
Practice adjusting each of the variables to get a feel for how each
will affect the water balance.
Set each variable back to its mean value when you are done.
Deficit — a negative balance, or
the amount needed to bring the
balance back up to zero.
Hint for question 3
On many newer computers, you
can use the Pointer Tool to find
the exact value of a bar in the bar
graph by holding it anywhere
over that particular bar. A small
window will appear with the exact
value.
. In the year , what would be the deficit in water availability (in
m) if the population growth rate increased from the current .
percent to  percent
a. with CAP water?
b. without CAP water?
. With a . percent rate of increase in water usage and a mean
annual precipitation of  cm, what would be the maximum
population growth rate that Tucson could support (with CAP
water) and still achieve a water balance in ?
. With a . percent rate of increase in water usage and a mean
annual precipitation of  cm, what would be the maximum
population growth rate Tucson could support (with CAP water) to
achieve a water balance in ?
. Which variable (population growth rate, rate of increase in water
use, or precipitation) do you think has the least influence on water
balance? Explain.
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Unit 4 – Water for a Desert City
Scientists have predicted a trend of hotter summers and drier winters in
the future.
. If Tucson’s annual precipitation drops to  cm per year, what
combination of population growth rate and rate of increase in
water usage would the city require in order to achieve a water
balance with CAP water in ? Find  combinations and enter
them in Table .
Table 1 — Maintaining a water balance with decreased precipitation
Combination
Population
growth rate
Water-use annual
increase
Annual
precipitation
%
%
cm
1
21
2
21
3
21
. Are the combinations of population growth rate and increase in
rate of water usage that you entered in Table  reasonable? Explain.
. List three ways that citizens or politicians could promote a lower
rate of population growth to help Tucson achieve a water balance
by .
a.
b.
c.
. Which of the three ideas listed above is most reasonable? Why?
Close the Excel spreadsheet. Do not save your work.
Quit ArcMap and do not save changes.
Water in the balance
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Water in the balance
Unit 4 – Water for a Desert City
Data Detectives: Where’s the Water?
Reading 4.3
Unit 4 – Water for a Desert City
The Tucson Basin aquifer
Aquifer basics
An aquifer is a formation of rock or sediment that is saturated, or
filled to capacity, with water. Knowing the geology of an aquifer and its
overlying sediments is important to understanding water infiltration
and recharge.
Unsaturated
zone
Water
table
Surface
water
Saturated
zone
(groundwater)
Impermeable
layer
Aquifer
Figure 1. Structure of a typical aquifer. Rivers and lakes form where the surface
drops below the water table.
The ability of water to infiltrate, or percolate down through the
sediments to reach the aquifer is dictated by permeability. If an aquifer
is permeable, water can move into it from overlying sediments. The
ability of water to be stored in an aquifer is determined by porosity. If an
aquifer is porous, it possesses empty spaces capable of retaining water.
As shown in Figure , the quality of an aquifer is determined by both
the permeability and porosity of the rock formation. Valuable aquifers
no
pore spaces
unconnected
pore spaces
connected
pore spaces
non-porous
non-permeable
porous
non-permeable
porous
permeable
Figure 2. High porosity and permeability are characteristic properties of productive
aquifers.
The Tucson Basin aquifer
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Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
are both permeable and porous, allowing water to flow through empty
spaces that are connected within the rock.
Basin and Range aquifers
The Basin and Range aquifers
Tucson is located in the Basin and Range Province, a geographic region
that extends from southern Idaho into Sonora, Mexico; and from eastern
California to central Utah (Figure ). This region is characterized by
mountain ranges that run north-south and are separated by low basins.
Viewed from the air, the Basin and Range region has been described as
looking like “an army of caterpillars marching toward Mexico.”
Tucson
Figure 3. Tucson, Arizona is
located within the Basin and
Range Province. The Basin and
Range network of aquifers are
shown here in blue.
Sediments eroded from the surrounding mountains fill the basins, and
can be from a few hundred to more than  m thick. Over millions
of years, water has infiltrated these sediment-filled basins, forming
the Basin and Range aquifers. In these aquifers, the depth of the water
table — the upper boundary of the aquifer’s saturated zone — varies
considerably, ranging from the surface (in flowing rivers and streams)
to nearly  m below the surface. The Basin and Range aquifer system
includes more than  independent watersheds covering over ,
km. These aquifers have played a critical role in the growth and
development of the southwestern U.S.
The aquifer in the Tucson Basin is composed primarily of sedimentary
rock including sand and clay. The cross-sectional diagram of the basin
below shows the composition of the sedimentary rock that fills the basin
and the historic and current location of the water table. In , the water
table was located in the sand layer, much closer to the surface. Since then,
it has dropped more than  m.
WEST
EAST
Elevation (m above sea level)
Santa Cruz
River
Water table
in 1940
Sand
Water table
in 2000
Sand & clay
Bedrock
Clay
Figure 4. Cross section of the Tucson Basin aquifer showing changes in the
water table from 1940 to 2000.
. What type of rock is associated with the Tucson Basin Aquifer?
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The Tucson Basin aquifer
Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
Recharging the aquifer
Water removed from an aquifer is replaced or recharged primarily
by precipitation. However, most of this precipitation is consumed by
other hydrologic-cycle processes and does not recharge the aquifer.
For example, much of the water becomes runoff, flows into drainages,
and is carried away from the area. Because Tucson’s water table is tens
to hundreds of meters below the surface in some places, streams and
rivers that form the drainage network are dry except during periods
of heavy precipitation. Water is also lost through evaporation from the
soil and absorption and transpiration by plants. Only  percent of the
precipitation falling in the Tucson Basin eventually recharges the aquifer,
although the process can take quite a long time. Scientists studying
recharge of the Tucson Basin aquifer have found that precipitation from
the s still has not infiltrated down to the main aquifer.
Colorado River water
In , the Tucson Water Department completed construction of a
facility where Colorado River water delivered by the Central Arizona
Project (CAP) canal can recharge the underlying aquifer. Once the water
infiltrates the surface soil and recharges the aquifer, it can be pumped
out of the aquifer and delivered to the city of Tucson. Eleven large
ponds, or basins, each ranging from approximately , – , m
( –  acres) in area are filled with CAP water. This water infiltrates
the soil under these basins in order to reach the aquifer underneath.
Since , over  million m (, acre-feet, or over  billion gal)
of water have recharged the underlying aquifer, and over  million m
(, acre-feet, or over  billion gal) of the newly recharged water
have been pumped out of the aquifer and delivered to Tucson since .
Water quality is constantly monitored in both the delivered CAP water
and water that has already recharged the aquifer. Since the new recharge
facility was constructed and started delivering recharged CAP water, the
need to pump groundwater from the Tucson Basin has eased and several
wells in the area have been put on standby status, meaning that they are
not currently being used but are available to pump if necessary.
A. K. Huth, The SAGUARO Project
Figure 5. Recharge basins filled with CAP water in the desert west of Tucson. The basins allow CAP water
to infiltrate the soil and recharge the underlying aquifer.
The Tucson Basin aquifer
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. What is the principal source of recharge in the Basin and Range
aquifers?
. How has recharging with Colorado River water changed Tucson’s
need for pumping groundwater from the Tucson Basin?
Keeping up with demand
It is important to balance the removal of water from the aquifer and
the rate at which the aquifer is being naturally or artificially recharged.
There are severe environmental consequences to withdrawing more
water than is recharged. When the water table drops, the aquifer may be
compressed by the weight of the overlying sediments, a process called
compaction. Compaction closes the empty pore spaces that previously
held water, reducing the porosity and permeability of the aquifer, thus
slowing infiltration and reducing the aquifer’s ability to store water. Once
this happens, the aquifer’s porosity and permeability cannot be restored.
Compaction of the aquifer due to groundwater pumping also results
in subsidence, or the lowering of the ground surface. Subsidence can
cause considerable damage to sewer, water, and gas pipes as well as
buildings and roads. Sewer pipes rely on gravity to maintain flow, so a
small change in slope due to subsidence can result in sewage backflow
— a serious problem! Intensive groundwater pumping can also drive up
water prices and decrease water quality. As existing wells are deepened
and new wells are drilled to reach the lower water table, the cost of these
upgrades are passed on to the consumer. More power is required to lift
the groundwater, raising energy costs. Water retrieved from deeper in
the Earth is also likely to be of lower quality because salinity increases
with depth.
. What problems can result from removing more groundwater from
an aquifer than the amount replaced by recharge?
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Investigation 4.4
Unit 4 – Water for a Desert City
Groundwater issues
Imagine coming home and finding cracks in your walls or, worse, that
your house is actually sinking. You and your home could be one of the
many victims of the over-pumping of groundwater. In this investigation,
you will determine the extent and severity of the effects of over-pumping
in the Tucson Basin.
The physical effects of subsidence
Launch ArcMap, then locate and open the ddww_unit_.mxd file.
Refer to the tear-out Quick Reference Sheet located in the Introduction to
this module for GIS definitions and instructions on how to perform tasks.
In the Table of Contents, right-click the Physical Impacts of
Subsidence data frame and choose Activate.
Expand the Physical Impacts of Subsidence data frame.
As you learned earlier, an aquifer is a water-bearing layer of rock in
which water fills the spaces, or pores, between sediment particles. The
water helps strengthen the rock against the pressure of the overlying
material. As water is pumped out of the aquifer, the weight of the
overlying material collapses the pore spaces and squeezes the sediment
particles together. As a result of this compaction,
• The porosity and permeability of the aquifer irreversibly decrease,
reducing the aquifer’s storage capacity.
• The ground surface gradually sinks, a process called subsidence.
The Compaction Movie
Click the Media Viewer button and choose the Compaction
Movie.
This animation simulates what happens when groundwater is removed
from an aquifer and the water table drops. Look for evidence of the
reduction in porosity and permeability, and of compaction.
. What would happen if you tried to recharge the aquifer again
after subsidence had occurred? Would you be able to fill it with as
much water as was there originally?
After viewing the movie several times, close the Media Viewer
window.
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Radar interferometry
This simulated cross section
through the center of the Tucson
Basin (dashed blue line in the top
image) shows recent subsidence.
Vertical distances are exaggerated.
Unit 4 – Water for a Desert City
Using satellite-based radar, scientists precisely measured the elevation
of the Tucson Basin floor in  and again in . Subtracting the 
elevation data from the  data produced the image you see in the
data frame on the computer, called a radar interferogram. It shows the
elevation changes that occurred over this -year period.
The elevation changes are color-coded, and appear as interference fringes,
similar to the rainbows you see on the surface of a soap bubble due to
tiny differences in the thickness of the bubble wall. In the interferogram,
each cycle of colored fringes — from one blue fringe to the next blue
fringe, for example — equals . cm of elevation change. The fringes
were then overlaid on a satellite photo in order to see city landmarks
underneath the interferogram.
Click the Media Viewer button and choose the Interferogram
Movie.
The Interferogram Movie shows a -D model of the ground subsidence
in the Tucson Basin. Elevation changes in the movie are greatly
exaggerated for emphasis.
View the movie several times, then close the Media Viewer
window.
Next, you will look at a cross-sectional view of the Tucson Basin.
Select the Cross section layer.
Using the Hyperlink tool , click on the blue cross-section line.
The window that appears shows a cross section of the Tucson Basin
aquifer and the location of the water table in  and .
. Describe the change in the water-table elevation from  to
.
. According to the cross section, where has the greatest change in
the water table elevation occurred relative to the river and higher
elevations?
. If the groundwater withdrawal continues at the same rate as it
has since , draw and label your prediction of the depth of the
water table in  on Diagram  on the following page.
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Diagram 1 — Water-table elevation
Elevation (m above sea level)
WEST
EAST
Santa Cruz
River
Water table
in 1940
Sand
Water table
in 2000
Sand & clay
Bedrock
Clay
Close the image window.
Turn off the Cross section layer.
Examine the Radar Interferogram layer.
. How many interference fringes do you see? (Count the rainbowcolored bands.)
Turn on the Radar Surface Change layer.
This layer summarizes the subsidence data shown in the radar
interferogram, with each interference fringe shown as a different color.
Click the Identify tool .
In the Identify Results window, select the Radar Surface Change
layer from the drop-down list of layers.
Using the Identify tool , click in each colored subsidence region
to collect data about that region. Read the maximum subsidence
and area from the Identify Results window.
. For each subsidence region, record the maximum subsidence that
occurred and the area of the region in Table  on the following
page. Round the area to the nearest whole number.
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Table 1 — Subsidence in the Tucson Basin
Band
Maximum
subsidence
Area
cm
km2
Rate of
% of city
subsidence affected
cm/yr
outer
middle
inner
Calculating rate
of subsidence
Rate of subsidence (cm/yr) =
Maximum subsidence (cm) ÷ 4
years
Calculating percent of city
Percent area =
(area [km2] ÷ 500 km2) × 100
Close the Identify Results window.
. Calculate the rate of subsidence (to the nearest tenth of a
centimeter per year) for each band over the -year period from
 –  and record it in Table .
. If the city of Tucson covers an area of about  km, calculate the
percentage of the city covered by each band (to the nearest tenth
of a percent) and record it in Table .
. If the subsidence rate remains constant, how much subsidence will
occur in the center of the basin (innermost band) between 
and ?
a. Calculate the number of years between  and .
b. Then multiply your answer from a by the subsidence rate
(cm/yr) of the inner band of the city.
The economic impact of subsidence
In addition to the physical impacts of over-pumping groundwater, there
are economic impacts as well. In this section, you will estimate the cost
of repairing damage to the sewer system caused by subsidence.
Collapse the Physical Impacts of Subsidence data frame.
Right-click the Economic Impacts of Subsidence data frame and
choose Activate.
Expand the Economic Impacts of Subsidence data frame.
The subsidence you calculated assumed that the population and water
use rate in the Tucson Basin will remain the same as it is today. This
layer shows the subsidence predicted by the U.S. Geological Survey for
the Tucson Basin between now and the year , based on expected
changes in population and water use.
Turn on the Sewer Damage layer.
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Select the Sewer Damage layer.
The Sewer Damage layer shows the locations of main sewer lines within
the predicted subsidence area. Gravity controls the flow in sewer lines,
so even small elevation changes can cause sewer lines to break, stop
flowing, or even flow backward!
Click the Statistics button .
In the Statistics window, calculate statistics for all features of the
Sewer Damage layer, using the Length (m) field.
Click OK. Be patient while the statistics are calculated.
The Total is the total length of sewer mains, in meters, within the
subsiding area.
. What is the total length of sewer mains within the subsiding area?
Round your answer to the nearest whole number.
Close the Statistics window.
When the ground in the center of the basin subsides . meters, about
 percent of the sewer mains need to be replaced. Next, you will
determine how much it might cost the city to replace these sewer mains.
To simplify your calculations, assume that subsidence across the basin
between now and  is . meters.
. Determine the length of sewer mains that must be replaced each
time the ground subsides . meters by multiplying the total
length of sewer mains in the subsiding area (from question ) by
% (.). Round your answer to the nearest whole number.
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. Determine the number of times the sewer mains will need to be
replaced by . (Hint: Divide the total amount of predicted
subsidence in  (. meters) by . meters (the amount of
subsidence that will require sewer main replacement).)
. Determine the total length of sewer mains that must be replaced
by  by multiplying the number of replacements (from
question ) by the total length of sewer mains that would require
replacement (from question ).
According to a local engineering firm, the cost of replacing a sewer main
is $/meter.
. How much will it cost the city to replace damaged sewer mains
between now and ?
Roads, sewers, and utilities are part of a city’s infrastructure, the basic
facilities needed for a city to function.
. In addition to the sewer mains, what other parts of Tucson’s
infrastructure do you think are affected by ground subsidence?
. Describe how each of these infrastructure issues could affect each
of the following groups:
a. Tucson Basin residents.
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b. Residents of the surrounding area.
c. Seasonal residents and vacationers in Tucson.
Quit ArcMap and do not save changes.
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Unit 4 – Water for a Desert City
Data Detectives: Where’s the Water?
Unit 4 – Water for a Desert City
Conserving water
Investigation 4.5
Water rates increase over time for several reasons. Rates must be raised
to cover the costs of operating, maintaining, and expanding the water
system, and paying employee salaries. Water rates may also be increased
to encourage customers to conserve water. The reasoning is that, as
water rates increase, customers will decrease their water use in order
to avoid higher monthly bills. In this section, you will investigate how
residential water rates changed over several decades in Tucson. You will
also determine how much water really costs.
Inflation — an increase in the
prices of products and services
over time. Inflation can also be
described as a decrease in the
purchasing power of money over
time. By “adjusting for inflation,”
economists can realistically
compare prices at different times.
Tucson’s water rates
The figures below show the population and water rates in the Tucson
metropolitan area over a century. The water rates in Figure  have been
adjusted for inflation to dollar amounts in the year .
Tucson Metropolitan Area population
1900 – 2000
Tucson Metropolitan Area water rates
1925 – 2000
1960
Figure 2. Tucson area water rates.
Figure 1. Tucson area population.
What does “Ccf” mean?
C = centrum (Latin: one
hundred)
Ccf = 100 cubic feet
= 2.83 m3
= 748 gallons
. According to Figure , approximately how large was the Tucson
Metropolitan Area population in ? How large was the
population in ?
a. .
b. .
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. Calculate the percent increase in population using the following
formula:
2000 population –1925 population
% increase =
× 100
1925 population
. According to Figure  (previous page), what was the water rate for
a  Ccf monthly usage in the Tucson Metropolitan Area in ?
What was the rate in ?
a. .
b. .
. Calculate the percent increase in the  Ccf-per-month water rate
from  to  using the following formula:
% increase =
2000 rate –1925 rate
1925 rate
× 100
. Is the percent increase in water rate more than, less than, or
similar to the rate of population increase? Why do you think this
is?
Table  on the following page shows the amount of water used by a
family of four to do common household tasks.
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Table 1 — Typical household water usage for a family of four
Task
Showering
Flushing toilet
Gallons
per day
80
100
Days
per month
30
30
Brushing teeth
8
30
Washing dishes
15
30
Cooking & drinking
12
30
Irrigation
48
30
Kitchen sink use
5
30
Laundry (1 load)
35
17
Gallons
per month
80 x 30 = 2400
Total gallons per month
. Calculate the monthly amount of water used by a family of four
for each of these tasks and record it in the last column of Table .
The first one has been done for you.
. Add the number of gallons per month used for all tasks in Table ,
and record the total at the bottom of the last column.
. Convert the total number of gallons per month to hundreds of
cubic feet (Ccf) per month. (Hint:  Ccf =  gallons.)
. Using the value you calculated for total Ccf per month in question
, and the water rates you identified in question , calculate what
a monthly water bill would have been in the years  and .
Use the following formula:
Monthly water bill for year = Ccf per month × water rate for year
a. .
b. .
. How much did the monthly water bill increase from  to ?
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. Calculate the cost per gallon of water in  using the following
formula.
Year 2000 cost per gallon =
Year 2000 monthly bill (question 9b)
Total gallons per month (Table 1)
. If bottled water costs $. per gallon, how many times more
expensive is bottled water compared to tap water? (Hint: Divide
$. by your answer to question .)
Imagine what it would cost to meet all of your family’s water needs with
bottled water!
. How do you think doubling water rates would affect the city’s
population and the city’s water supply? Explain.
. Do you think increasing water rates is a good way to encourage
people to conserve water? Explain.
Stop the pumping!
You have investigated subsidence and its potential for damaging homes,
water mains, and sewer lines. In theory, the solution to many of these
problems is easy — stop pumping groundwater from subsiding areas.
However, what effect would this have on Tucson’s water supply?
Launch ArcMap, then locate and open the ddww_unit_.mxd file.
Refer to the tear-out Quick Reference Sheet located in the Introduction to
this module for GIS definitions and instructions on how to perform tasks.
In the Table of Contents, right-click the Wells data frame and
choose Activate.
Expand the Wells data frame.
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This data frame shows the major streets in the city of Tucson. The
pink area represents the area where significant ground subsidence is
occurring.
Turn on the Wells layer.
Active Management Areas
The Tucson Active Management
Area (TAMA) is one of five active
management areas in Arizona,
designed to protect groundwater
resources within those areas. The
boundary of TAMA is roughly the
boundary of the groundwater
basin.
This layer shows the locations of drinking-water wells in the Tucson
Active Management Area.
Click the Select By
Location button .
In the Select By
Location window,
construct the query
statement:
I want to select
features from
the Wells layer
that intersect the
features in the Radar
Subsidence layer.
Click Apply.
Close the Select By
Location window.
The drinking-water wells
within the subsidence zone
will be highlighted.
Click the Statistics button .
In the Statistics window, calculate statistics for only selected
features of the Wells layer, using the Production (gal/year) field.
Click OK. Be patient while the statistics are calculated.
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In the Statistics window, the number of wells in the subsidence area is
the Number of Records, and the total annual production (gal/yr) of
these wells is the Total. (Hint: You may need to increase the size of the
Statistics window to see all the results.)
. How many drinking-water production wells (Number of
Records) are located within the central Tucson subsidence area?
. If the wells within the subsiding area were removed from
production, how many gallons (the Total) would no longer be
available for human use?
. How do you think the city might make up for this shortfall?
Close the Statistics window.
Click the Clear Selected Features button
.
Water conservation
Research has shown that only about  percent of the precipitation that
falls in the Tucson Basin is recharged into the aquifer. A significant
amount of precipitation runs off into storm drains and sewer systems.
In this section, you will calculate how much water can be harvested, or
captured and used, from a small area during a single summer storm.
Collapse the Wells data frame.
Right-click the Summer Harvest data frame and choose Activate.
Expand the Summer Harvest data frame.
This data frame shows an aerial photo of the University of Arizona
campus, located in central Tucson. The blue features are University
buildings. The University Buildings layer contains information about
the area covered by each building, in square meters.
Click the Statistics button .
In the Statistics window, calculate statistics for all features of the
University Buildings layer, using the Area (m^) field.
Click OK. Be patient while the statistics are calculated.
In the Statistics window, the total area covered by the buildings is the
Total.
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. What is the total area covered by campus buildings, in square
meters?
. Following the directions below, calculate how much water could
be collected from the rooftops of the buildings on campus if the
storm produced . cm ( in) of rain.
a. Convert centimeters to meters of rain ( cm =  m).
Watch a summer
thunderstorm
USDA/ARS/Southwest Watershed Research Center
b. Calculate the volume of rain on the rooftops, in cubic meters.
(Hint: Volume = area of rooftops [m] calculated in question 
× depth of water [m] calculated in question a.)
To view time-lapse movies of
summer thunderstorms, click
the Media Viewer button and
choose Thunderstorm Movie 1, 2,
or 3. (Don’t spend too much time
watching the storms — you still
have work to do.)
c. Convert the volume of water collected from the rooftops to
gallons. (Hint: Multiply your answer from question b by 
gal/m.)
. How could water be collected from the rooftops of the buildings
in the main campus area?
. For what purposes do you think this harvested water could be
used?
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. How does the volume of water that can be harvested from
campus rooftops (question c) compare to the mean annual
well production in the subsidence zone (question )? Write your
answer in terms of a percentage of annual well production. (Hint:
Divide the answer to question c by the answer to question 
and multiply the result by .)
Hint for question 23c:
Remember that 1 cubic meter
(m) is equal to approximately 264
gallons.
. If the roof area of an average-sized home is  m, how much
water could be collected from the roof of an average home in the
same storm?
a. Convert . cm of rainfall to meters.
b. Multiply your answer to a by the roof area to get the volume
of water in m.
c. Convert your answer to b to gallons.
. Referring to Table , determine what percentage of the average
family’s monthly irrigation needs could be met using the water
harvested from the rooftop in this storm. (Hint: Divide the
answer to question c by the number of gallons used for monthly
irrigation [Table ] and multiply the result by .)
. Do you think this is a reasonable technique for conserving
water in Tucson? What issues might complicate this type of
conservation?
Quit ArcMap and do not save changes.
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Wrap-up 4.6
Unit 4 – Water for a Desert City
The voice of conservation
Voicing your ideas on conservation
Now that you have examined the environmental, physical, and economic
issues surrounding Tucson’s water situation, you may have developed
some of your own solutions to the water supply problem. This section
provides you with an opportunity to express your ideas and develop
a conservation plan for the city of Tucson, using the knowledge you
gained in this investigation.
Project
You have been hired by Tucson Water as a community water
conservation specialist. Your job is to communicate with citizens about
ways to conserve water and the importance of doing so. Write a letter
that will be mailed to all residents, explaining why they should care
about conserving water and what they can do to save water. In this
letter, describe three different action plans by which residents can help
decrease groundwater use in Tucson. Give specific facts and details
about your plans, about what will happen if residents do not conserve,
and about the benefits they will enjoy if they do conserve. You might
want to refer to values you calculated in Investigation .B (in the
Tucson Water Balance Sheet) and Investigation ..
The voice of conservation
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